Reverse recovery

[Tim Williams]

Why is reverse recovery time always measured at If = 1A? This is an
insanely arbitrary value.

Tim

 

[Tim Williams]

Unfortunately the absurdity doesn't seem to extend to signal diodes, a
quite delicious absurdity as they wouldn't even withstand the 1A, at least
not continuously. It does appear to extend in the other direction,
despite arbitrarily powerful diodes.

I am hard pressed to find a *power* diode which differs. Rarely, graphs
at rated current are included at least. I did not look at large modules
or hockeypucks, which don't seem to be listed on Mouser, at least not
"ultrafast" types.

10A 200V, If = 0.5A, no graphs
http://www.mouser.com/catalog/specsheets/byq28e.pdf

16A 200V, If = 0.5A, no graphs
http://www.fairchildsemi.com/ds/FE/FES16DT.pdf

15A 200V, If = 1A, with graphs
http://www.st.com/stonline/books/pdf/docs/12364.pdf

15A 1200V, graphs. Seems to me, IXYS at least has nice graphs, if only
they'd use useful numbers up front.
http://ixdev.ixys.com/DataSheet/L187.pdf

50V 30A, If = 0.5A, no graphs
http://www.mouser.com/catalog/specsheets/fep30.pdf

600V 120A, If = 0.5A, with graphs
http://www.st.com/stonline/books/pdf/docs/10766.pdf

600V 280A, If = 1A, with more rows and graphs
http://www.mouser.com/catalog/specsheets/94067hfa.pdf

A sufficiently representative sample?

Tim

 

[Tim Williams]

I can certainly see your point with the ones that are rated much above
10A -- not to mention 100A!

The real question is -- how much current does it take to make sure that
the diode is saturated with minority carriers, thus making the reverse
recovery time measurement representative of what will happen at rated
current?

What's even more bizarre, if they do that on the really big diodes
(>100A), capacitance will be so huge relative to the 1A reverse current
that they'll look even *slower*! Same as calling a schottky "4ns" or
something, which is based only on junction capacitance and test current.

Presumably, all junction diodes use approximately the same current density
(allowing for differences in voltage and speed), so they'll be fairly well
saturated at rated current. Somewhere up the knee, where they start
looking resistive (1~2V).

The diodes that provide a graph usually have parameters of If, dI/dt and
Tj, and end up in the 200ns range for something like dI/dt-snubbed hard
switching (like CCM buck/boost). It would be nice if they'd just use that
number to begin with.

Tim

 

[legg]

Why is reverse recovery time always measured at If = 1A? This is an
insanely arbitrary value.

Tim

for a particular diode the relationship between stored charge and
forward current or di/dt is a direct one. Reverse recovery time shows
an inverse relationship to Ir/If in current-contrled situations.

Jedec published data for early fast rectifiers included a typical test
circuit drawing and graphs illustrating typical stored charge data for
different junction temperatures, forward current and di/dt. The
relationship to temperature may be non-linear; increasing with
temperature to a point where it begins to reduce and is process
dependant.

While the one amp recovery time spec will often be quoted,
larger-rated devices will often include data for at least one other
higher current level.

Jelly-bean components will be supplied with jelly-bean paperwork and
should only be applied where jelly-beans will function adequately. A
good designer will work with what's available.

RL